1. Field of the Invention
The present invention relates to a high-strength spring steel used in automobiles, aircraft equipment, various types of industrial machinery, and so forth.
2. Description of the Related Art
In an effort to improve fuel economy, there has been an urgent need in recent years for weight reductions in automobiles. These reductions are required of many different parts, with suspension parts being no exception. One way of handling this is to set a higher design stress for suspension springs. Specifically, it is effective to increase the strength of springs. Si-Mn-based SUP7 and Si-Cr-based SUP12 are the main types of suspension spring steel in use at the present time, but further increases in design stress will require higher strength than with these types of steel. The strength of a steel material is generally closely related to its hardness, but there was concern that increasing the hardness of spring steel would lower its toughness. Specifically, diminished toughness was an inevitable consequence of achieving hardness over that of current spring steel. In increasing the strength of suspension springs, toughness also had to be greater than that of current steel to ensure reliability in these springs.